SKOWHEGAN, Maine — Members of Rita St. Peter’s family celebrated and cried Thursday afternoon after hearing the jury convict an Industry man of murdering the 20-year-old Anson woman.

Jay S. Mercier, 56, was found guilty of murder by a jury of seven women and five men at Somerset County Superior Court at 2:45 p.m. The verdict brought closure to a crime that occurred more than 32 years ago. St. Peter’s bloody, beaten body was found off Campground Road in Anson on July 5, 1980.

Mercier, who rarely shifted in his seat or consulted with his attorneys during the trial, showed little emotion after the verdict was read. He will likely be sentenced in November, said Superior Court Justice John Nivison.

The jury took less than three hours to reach its verdict.

Ten members of St. Peter’s family were jubilant on the steps of the courthouse after the trial ended.

“I know [Rita’s] up there somewheres, just smiling,” said St. Peter’s sister Christine Belangia of Weld. “Justice has prevailed for Rita St. Peter in 2012.”

Belangia read a statement on behalf of the family, thanking the Maine State Police and crime laboratory for their work — especially Detective Bryant Jacques, who re-examined the case in 2005.

“He’s awesome,” Belangia and several members of the family said.

Assistant Attorney General Andrew Benson, who prosecuted the case for the state, also commended the police.

The trial provided unusual challenges, he said.

“Because of the age of the case, it was a difficult trial,” Benson said outside the courthouse.

Missing evidence, witnesses who had passed away and faulty memories of witnesses were among the difficulties. Benson said the most important pieces of evidence were there, however.

Benson said he believed the DNA evidence played a pivotal role in getting a conviction for murder.

“I think the critical piece of evidence in this case was the fact that Ms. St. Peter had obviously just had a sexual encounter because of the nature and the disarray of her clothing,” said Benson. “And the fact that Mr. Mercier’s DNA — his sperm — was not in her underwear, only in her [body].”

Jacques initially was able to link Mercier to the smear slides taken from St. Peter’s body in 1980 through a discarded cigarette butt that Mercier left on the ground during an interview with the detective in Industry in January 2010. DNA was taken from the cigarette and matched to DNA taken from St. Peter’s body.

It took 30 years to make that connection, but Benson expressed satisfaction at the result.

“I think the wheels of justice are working excellently,” the prosecutor said.

One of Mercier’s attorneys, John Alsop of Skowhegan, said the DNA evidence was hard to overcome.

“Obviously, it was the presence of his sperm in her private parts and his denial of any involvement” that were especially difficult, Alsop said after the trial. “That was the new evidence that came out. That’s why they brought the case forward after 32 years. It was pretty potent stuff.”

Alsop said he hadn’t had a chance to speak with Mercier immediately after the verdict was read. He also didn’t indicate if there were any plans for an appeal.

“We’ll jump off that bridge when we get to it,” Alsop said of the potential for an appeal.

Alsop had tried to argue in his closing statement Thursday morning that mill construction workers — not Mercier — may have sexually assaulted and killed St. Peter. However, Benson wasn’t buying it.

“Mr. Alsop will have you believe that Mr. Mercier is the unluckiest person on Earth,” said Benson.

After about 2½ hours of deliberation, the jurors asked the judge to instruct them again on the definition of the lesser charge of manslaughter.

Neither Benson nor Alsop read much into the jury’s asking for clarification.

“Notes are inscrutable,” said Benson, referring to jurors’ requests for clarifications from a trial judge. “Twenty-five years ago, I thought I was able to interpret them. Today, I realized I was completely incapable of interpreting it. You never know the basis of a note. You don’t know if they’re getting ready to compromise on a manslaughter, or if they’re trying to convince someone it’s not a manslaughter.”

Fifteen minutes after the clarification was given, the jury reached its verdict.

After the jury forewoman read the verdict aloud, Alsop requested that the jurors be polled individually. Each of the 12 jurors was asked, “Guilty or not guilty?” and each answered, “Guilty.”

Nivison said Mercier will continue to be held without bail until sentencing.

The verdict brought an end to the wondering and speculation that have haunted the St. Peter family for three decades.

“We can now have closure to the mystery of that night of July 4, 1980. Rita can rest in peace, knowing that justice has finally prevailed,” Belangia said.